Thermoelectric power generation allows direct conversion of heat into electricity without moving parts and is inherently reliable. The heat source is not restricted to any type of fuel or process and must only provide input thermal energy to create the required temperature differential between hot and cold sides of the generator. Prior art techniques for implementing the well known Peltier and Seebeck effects have been able to demonstrate direct conversion of heat into electrical power. A disadvantage of prior art techniques is the low conversion efficiency. Since a proposal in 1958 for inter-metallic semiconductors in application to thermoelectric conversion, such as bismuth telluride (Bi2Te3), the efficiency has been limited to less than or equal to η˜5% up until the present time. This level of efficiency is useful only for niche applications in solid-state cooling and space flight power generation based on radioisotope heating. The current status of thermoelectric machines for direct conversion of heat energy into electricity conversion technology is briefly reviewed.
Seebeck and Peltier Effects
Mobile charge carriers in solids possess negative or positive charge and carry electric current under the influence of an electromotive force. The large numbers of electrons in a metal or electrons and/or holes in a semiconductor at thermal equilibrium can also carry heat and entropy. A temperature differential across a solid provides an energy gradient such that charge carriers flow from a hot side to a cold side so as to create an electric current. This implies a coupling between thermal and electrical phenomena, which is generally called a thermoelectric effect. The Seebeck effect and the Peltier effect are based on the above process and are well known to workers in the fields of semiconductors and physical sciences. Generally, the Seebeck effect is a phenomenon wherein a voltage, ΔV, is induced in proportion to applied temperature gradient, ΔT. The Peltier effect is a phenomenon wherein heat absorption/emission, Q, is induced at the junctions of an applied current. In the presence of the coupling between thermal and electrical phenomena, it is, in principle, possible to convert heat into electric energy, and vice versa. An immediate advantage of thermoelectric devices is the absence of physically moving friction loss parts. Furthermore, thermoelectric devices do not produce waste matter through the conversion process, which can be implemented at the micro (10−6 m) and/or nano (10−9 m) scale, and can therefore, for example, be implemented into electronic devices.
Thermoelectric Figure-of-merit & Applications
A figure-of-merit (FOM) for thermoelectric materials is given by Z=S2σ/κth, where S=ΔV/ΔT is the Seebeck co-efficient or thermoelectric power; σ is the electrical conductivity and κth is the thermal conductivity. Typically the dimensionless quantity ZT is used, where T is the operating temperature. The thermal to electrical conversion efficiency is defined as:η=(TH−TC/TH)[(1+ZT)0.5−1]/[(1+ZT)0.5+TC/TH]  (1)
where, TH (TC) is the hot (cold) side junction temperature. The prefactor (TH−TC/TH) is the Carnot efficiency reduced by the FOM factor in [ ] brackets. Therefore, η can be considered the fraction of Carnot efficiency. To attain 50% Carnot efficiency using operating temperature gradients TC/TH≧0.3 requires FOMs in the range 4<ZT<6. Clearly, there is a need to improve thermoelectric FOM for power generation devices. Using the Peltier effect, the thermoelectric device can cool materials. It should be emphasized that thermoelectric cooling does not need the action of exchange media, such as a Freon gas, and is therefore an alternative for a Freon-gas refrigeration if thermoelectric efficiency can be increased beyond prior art. Another advantage is heating and cooling cycles can be quickly changed via the applied current direction. Thus thermoelectric refrigeration can also be used in closed loop temperature control providing hot and cold stimulus.
Using the Seebeck effect, thermal energy or heat can be converted into electric energy. FIG. 2 shows the schematic process of the thermoelectric power generation. When the left side of the sample is heated, a thermoelectric voltage is induced in proportion to the temperature difference. If a load is connected to the sample, the electric power is consumed at the load. For constant heat energy input a constant thermoelectric current is generated, capable of providing electrical power to an external load, R, such that PL=I2R; optionally, R may also be a complex impedance for inductive and capacitive loads). Alternating current can thus be produced via dc-to-ac conversion devices. Alternatively, a sinusoidal modulated thermal source can be input to a thermoelectric converter designed to have an appropriate temporal electrical response such that the input modulation is transferred upon the electrical output. If, the time response of the thermoelectric device is slower that the input thermal modulation, for example by the use of short time pulse and repetitive thermal energy source, then the thermoelectric device will filter the high frequency components so that the repetition frequency can approximate an alternating current. This concept may be of use in conjunction with pulsed fusion reactors for waste heat recovery.
Advantages of thermoelectric power generation are (i) electric power source without moving parts and maintenance, (ii) energy recovery from waist heat generated by conventional energy sources (e.g. fossil fueled plants), (iii) high efficiency direct thermoelectric conversion from non-fossil fuel driven energy sources (eg nuclear and geothermal plants), and (iv) long plant operating lifetimes. Recently, waste heat recovery and conversion to electricity is becoming practical for application to improving combustion engine efficiencies in smaller scale systems such as automotive engines. In general, a typical combustion engine develops only 25% useful shaft rotation power with as much as 40% waste heat energy via exhaust gas. The temperature rise of a typical exhaust gas manifold is capable of attaining extremely large temperature differential with reference to the ambient temperature. This heated manifold could be used as an appropriate temperature source for the present invention.
Similarly, electricity grid generation plants consume fossil or nuclear fission fuels to heat a suitable high thermal capacity fluid or medium so as to generate steam or pressure in the said medium thereby generating a means to drive a mechanical turbine or rotation device. The said turbine shaft is connected to a suitable electromechanical alternating and/or direct current generator. Typical conversion efficiencies from thermal to electrical energy in such systems are on the order of only ˜35%. Therefore, there would be considerable cost, environmental and safety gains for direct energy conversion of waste heat from a thermal source into electricity.